This invention relates to homodyne optical coherent receivers which are phase insensitive.
Coherent optical systems using an optical local oscillator (OLO) at the receiver can give receiver sensitivity approaching the quantum limit. The actual sensitivity depends upon the modulation format and the receiver type. A heterodyne receiver, in which the signal and OLO are set to have a constant difference frequency (IF), offers 3 dB worse sensitivity than a homodyne receiver with the same modulation format. A heterodyne receiver requires that the LO and signal are synchronized in frequency but not in phase: all possible relative phases are explored over each period of the IF. The receiver needs sufficient bandwidth to deal with the IF with modulation sidebands on each side. For wideband data signals high IF frequencies are required (e.g. approaching 1 GHz for 565 Mbit/s), and so very wideband special receivers are required. On the other hand, homodyne receivers offer higher sensitivity, and provide the data signal at baseband. Consequently, receivers already developed for direct detection systems can be used. These would generally have lower noise than the equivalent heterodyne receiver; in coherent operation, shot noise limited operation would then be achieved with lower OLO powers. However, in order to achieve these advantages, the OLO needs to be phase locked to the incoming signal; if the OLO and the signal are in quadrature than their coherent product is zero and no modulation can be detected. Optical phase locking has been demonstrated, but it requires a device with fast phase response in the optical feedback loop. Moreover, transient disturbances to the transmission path could cause large phase deviation, and the phase synchronization could be knocked out of lock. In fact, the phase length of an optical fibre is very sensitive to external perturbation, and hence the growing interest in fibres as sensors of temperature, acoustic vibration etc. An optical phase locked loop will not be a robust and practical system.